Universal Serial Bus (USB) provides both data and power to downstream devices. Under the standard, each USB interface includes two data lines, plus power and ground. The maximum power available under USB for a downstream device is 500 mA@5V (2.5 W). Any device demanding higher power must use an external power source, such as a power brick, eliminating some of the simplicity and advantage of a single cable connection for both power and data.
USB+ was developed as a way of providing additional power to external devices without the use of an external power brick. A USB+ connector defines four additional power pins, providing an additional ground conductor and up to 6 A of +5V, +12V and +24V power. In contrast to the 2.5 W available from standard USB, a single USB+ connector can, therefore, provide up to 144 W of power.
USB ports are standard on any new motherboard today. However, USB+ ports typically are added as either an add-on card to the PC (e.g., a PCI card) or as a standalone USB+ HUB with its own power supply.
In case of the external HUB, the external power supply provides the power distributed to the attached devices via the USB+ connector.
In case of a PCI add-on card the power typically comes from the internal power supply of the PC. The +5V or +12V comes directly from the internal power supply and the +24V is boosted from the +12V.
Today's competitive PC marketplace demands that the size of the internal power supply of the PC be in line with the projected maximum internal power consumption. Installing a larger power supply than required would increase the-cost of the PC. PCs do not, therefore, typically have a great deal of extra power capacity, at least in the low-end units. Therefore, a typical power supply will not have sufficient reserve (unused) power available to provide the up to 144 W that could be required for external USB+ devices.
In addition, while the sum of the reserve power of the power supplies on all different voltage outputs may be sufficient reserve power, it may not match the power requirements. That is, reserve power may be available on the +5V rail where the demand may be needed on the +12V or +24V or vice-versa.
Finally, power demand, and the amount of power that a power supply can provide, vary as a function of time, temperature and operating mode. A PC can be reconfigured by adding either external or internal devices, activating or deactivating high-powered functions.
What is needed is a system and method for routing power to external devices that addresses the issues raised above and other issues that will become apparent in reading the following description of the present invention.